Here is the original article, which has slightly more info:

They "checked to make sure the flight wouldn’t be dangerous or illegal", and they used forecasts from "a website that calculates a weather balloon’s estimated landing spot based on input launch coordinates, prevailing winds, and balloon specs" to time the launch so that the balloon would land in Canada in the US.

http://www.thestar.com/news/article/1120808--toronto-teens-s...